Cathode structure for electric discharge devices



CATHODE STRUCTURE FOR ELECTRIC DISCHARGE DEVICES Filed Nov. 16, 1952 Inventor- LeRog E. Record.

y MM MQ His Attorney.

LE ROY E. RECORD I 2,036,605

Patented Apr. 7, 1936 unrrso STATES CATHODE STRUCTURE FOR ELECTRIC DISCHARGE DEVICES Le Roy E. Record, Scotia,

N. Y., assignor to General Electric Company, a corporation of New York Application November 16, 1932, Serial No. 642,890

10 Claims.

The present invention relates to electric discharge apparatus and more particularly to the construction of cathodes employed in devices of this sort.

Itis the usual practice to employ indirectly heated cathodes of a rugged type as the source of electrons in large arc discharge tubes. These cathodes consist generally of a wire helix adapted to be heated and contained within a metal casing which may be coated with an electron-emitting material so as to constitute the cathode proper. There may be secured to the casing in any suitable manner a number of transversely-extending members, usually in the form of disks which are also coated with electron-emitting material.

These disks are preferably equi-distantly spaced along the length of the casing. In addition, there may be provided one or more perforated cylinders surrounding the outer peripheries of the disk in order to conserve the heat of the cathode. The size of the openings in the cylinder is determined by the maximum current it is desired to pass between the cathode and the anode. In addition to allowing the readyegress of electrons, and the ingress of positive ions, these openings necessarily cause a certain amount of the heat to be lost by radiation. Devices of this general character are employed for translating relatively large amounts of current, particularly when the gas or vapor pressure within the device is sufficient to support an arc discharge. The starting of the discharge may, if desired, be controlled by an electrostatic member or grid interposed between the cathode and the anode, as is well known in the art.

When devices of this sort are operated, the current at low current densities has been found to concentrate into paths which extend through a limited number of perforations in the heat shield rather than to distribute itself over the entire shield and to pass through all of the openings. In case heat shields are not used, so that the entire surface of the cathode is exposed directly to the anode, the arc discharge has been found in some cases to concentrate itself about a localized portion of the cathode. In either case, the severe bombardment and heating of the localized region of the cathode by the impinging arc stream causes the electron-emitting material to leave this region, thereby greatly shortening the operating life of the cathode as a whole.

The reason for the tendency of the arc to concentrate in a localized region probably resides in the fact that the cathode is hotter in this region than at the remaining portions, consequently offering less potential drop and greater inducement for the current to strike the cathode at a spot of limited size. This unevenness of temperature over the cathode surface is due to the fact that certain portions of the surface lose their heat at a much faster rate than other portions. 5 Paths for heat transfer by conduction are readily provided through the supports which are generally secured at one end to the heat shield and at the other end to a reentrant stem of the envelope, also by the cathode leading-in conductors which comprise metal members of large cross section in case large heater and load currents are employed. There is also considerable heat lost by radiation. The facility with which heat transfer is provided by the cathode supports and leading-in conductors is not equally distributed throughout the entire cathode structure so that the remote portions of the cathode do not have the same opportunity of relieving themselves of heat as the portions nearer these supports and leading-in conductors. In cathodes of large metal content designed for power tubes, the temperature difference between the center and ends of the cathode at normal operating voltages may be very considerable, in fact, in a typical form of tube this has been found to be as much as 150 C.

An object of the present invention is to provide an improved cathode construction which will run at an even temperature over its entire surface notwithstanding different heat losses by conduction and radiation from respective portions of the surface. A more general object is to provide a cathode over which the anode current distributes itself uniformly, thereby causing the cathode to have a long operating life and enabling the device as a whole to handle greater power. In brief, these objects are attained in one of two ways or a combination of them (1) by distributing the heating power of the heater so as to compensate for the variation of heat losses throughout the cathode, and (2) by decreasing the heat losses in the cooler portions of the cathode and increasing them in the hotter portions. In carrying out the first of these methods, an improved form of heater is provided in which the individual turns of the heater helix are spaced unequal distances apart, thereby changing the distribution of the heat supplied to the cathode. In accordance with the second method, the invention contemplates the use of an improved form of heat shield which contains openings of different sizes, thereby to regulate the amount of heat deliberately radiated and to compensate for that lost by conduction and unavoidable radiation.

The invention w ll be better understood when reference is made to the following description and the accompanying drawing in which Fig. 1 shows an elevational view of an arc discharge device provided with the improved cathode, in part section, so as to show the latter. The cathode is broken away to show the relation between the heater, cathode and heat shields. Fig. 2 is an enlarged elevational View of the heater and its support, while Fig. 3 is an elevational view of a modified form of cathode structure.

Referring more particularly to Fig. 1 of the drawing, numeral .9 designates a cylindrical metal envelope, of copper, iron or the like, which terminates at the top in a handle-like extension 2, and at the bottom, is closed by a glass member (not shown) joined to the metal a knife-edge or a tapered seal. The metal part of the envelope constitutes the anode of the device and is supported within a large metal casing 3 of cylindrical form and fabricated of thin metal. This casing serves conveniently as a base for the tube on which the latter can rest when it is not being used, also as a protection for the seal-off and glass parts constituting the lower part of the envelope.

The envelope 5 contains a cathode of an improved form as will be described presently, and

ismounted within the envelope by means of several equi-distantly spaced metal supports 4 of channel construction, secured at the lower end to a clamp which embraces a reentrant stem (not shown) formed in the glass portion of the envelope. Leading-in conductors 5, which usually take the form of flexible cable, are provided for energizing the cathode. These conductors pass through the lower glass portion or the envelope and through the metal casing 3 to the exterior.

The cathode is shown, by way of example, as being of the indirectly heated type and consists essentially of a heater 6 contained within a metal casing I. The heater is preferably made of tungsten Wire wound as a helix on a rod 8 of insulating material, such as alumina (A1203) The member 8 has a central longitudinally-extending bore through which a metal rod is driven for increasing the rigidity of the member. The lower end of the heater is secured in any suitable manner to one of the flexible conductors 5 and the upper end is welded to metal plate secured to one of the heat shields, as will be explained more fully hereinafter. There may be provided a number of metal extensions, extending either transversely longitudinally. of the casing. These extensions may conveniently take the form of metal disks 9 spaced equi-distantly apart and secured in any suitable manner to the casing. These disks are mounted on metal rods (not shown) provided with spacer members it, and are preferably coated with electron-emitting material capable of giving a high degree of electron emissivity.

If desired, the cathode may be provided with one or more, three as illustrated, metal cylinders E I, which surround the disks and are spaced apart in any suitable manner, for example, by indentations l2. These disks are closed at the top and bottom by metal members is and if desired, an additional metal cap M may be provided at the upper end, as shown. One of the metal members l3, at the top and bottom of the cathode, supports a block of insulating material !5 counterbored to receive opposite, ends of the insulating rod 8. The cylinders H contain a number of openings l6, preferably of circular shape and of the same diameter, equi-distantly spaced over the peripheral surface of each cylinder. The openings of the respective cylinders are arranged to register with one another so that the anode i can see the cathode through each set of openings. The cylinders H serve the purpose of conserving the heat of the cathode, thereby enabling the latter to operate at optimum eiiiciency, while the openings it provide a path for the current, as is well known. A combined electrostatic and heat shield H in the form of a disk with a bent underlip portion may be provided at the lower end of the outer heat shield. The support rods 4 preferably extend along the entire length of the outer heat shield and are secured to the latter at positions between the openings.

The envelope may be either highly evacuated or contain inert gasor vapor, for example, mercury vapor, at a pressure sufiiciently high to. neutralize space charge effect, thereby to support an arc or a glow discharge at practical voltages.

The operation of a tube of this kind is well known and it is suflicient to: state that when alternating voltage is applied between the anode and cathode, an arc discharge passes between the electrodes during each positive voltage half cycle so that direct current is obtainable from the device. In case a grid member is provided between the cathode and anode, the member may be biased by alternating or direct current to control the average rectified current in the output circuit.

When the tube is operating with a high power output so that the current flow takes on the characteristics of an are or glow, the are tende initially toconcentrate about a few openings in the heat shield rather than spreading itself over the entire apertured surface. This concentration is obviously undesirable in that the arc attacks a localized portion of the cathode instead of diffusing over the entire cathode surface and in a short time causes a complete evaporation of the electronically active coating on the localized cathode portion. The cathode is therefore normally subject to quick deterioration. This undesirable current concentration is believed to be caused by the unevenness of temperature over the cathode surface, so that the hotter portions thereof tend to emit a greater number of electrons than the cooler portions, thereby drawing excessive current to these hotter portions. These differences of temperature are produced for the most part by the diflerences in the amounts of heat carried away from the cathode by conduction or otherwise through the various parts of the cathode, the support rods and leading-in conductors, etc. An inspection of the cathode as illustrated will show that the ends of the cathode are subject to greater cooling effects by way of metallic conduction than the center of the cathode and that the lower end of the cathode (as shown) tends to become slightly cooler'than the upper end, due to the closer proximity of the reentrant stem to which heat may be conveyed through the support members 4 and the leading-in conductors 5.

In order to offset or compensate for the differences in heat ioss underthe conditions noted, I propose, in accordance with my invention, to supply heat to the various portions of the cathode in proportion to the amount of heat lost. heat-compensating eifect is conveniently obtained in the case of a helical heater by spacing the turns of the heater unequal distances apart, as shown more clearly in Fig. 2. It will be noted that the spaces between the middle turns are much larger conditions, I may use a than the spaces at the ends of the heater and furthermore, that the spaces at the lower end of the heater are slightly less than those. at the upper end of the heater. A heater of this improved construction provides an elongated source of heat, the intensity of which is gradated over its length in such a manner as to compensate for the deficiency of heat in the respective portions of the casing l3 and associated disks 9 which is lost by the respective portions by conduction. In the prior forms of helical heaters, the turns are equally spaced, and consequently, supply the same amount of heat to all portions of the cathode. It has been found that when the turns of 'the 1 heater helix are spaced to give the desired gradation of heat, it is possible to reduce the temperature difference between the center and ends of the cathode from an initial difference of 150 C. to practicall zero.

While I have shown the heater turns as being more widely spaced in the center than at the ends and being closer together at the bottom than at the top for reasons stated, it will be understood that my invention is not limited to this arrangement but contemplates the broad use of an extended or elongated heat source, the heat intensities of the various portions of which are controlled by the configuration, dimension,

or relative positions of the respective heater portions, to compensate for the heat losses of the cathode regardless of the location and manner of these losses.

Instead of controlling the intensities of respective portions .of a heater in order to obtain an even cathode temperature under operating heater of conventional design, 1. e. having the turns of the helix equidistantly spaced, and control the rate at which heat is radiated from the respective portions of the cathode. The amount of heat radiated by a given cathode portion is made to correspond and to compensate for the amount of heat unavoidably lost by that portion. Thus in Fig. 3 the heater 6 comprises a wire helix wound on an insulating member 3 and having the turns equidistantly spaced over the length of said member. Instead of the heat shield ll having openings of substantially the same diameter as in the case of Fig. 1, these openings are now of different sizes, or if desired, of different shapes. For example, in the middle of the heat shield, the circular openings l6 are shown of much larger diameter than the openings at each end of the cathode, the openings being gradated in size or shape to correspond with the respective amounts of heat lost by portions of the cathode. As was explained in connection with Fig. 2, wherein a greater amount of heat was supplied to the lower end of the cathode than at the upper end thereof due to greater heat losses, in Fig. 3, the openings at the lower end of the heat shield are substantially smaller than those at the upper end in order to reduce the amount of heat radiated from the lower end. The relative sizes of the openings it are as indicated under the specific conditions noted, and their actual sizes may, of course, be determined by the current-carrying capacity of the device and other factors. The sizes of these openings in aggregate may be made to conform to the aggregate size of the openings of uniform diameter found in the conventional type of heat shields, so that the gradation of these openings produces no undue voltage drop in the tube.

While I have shown two methods and apparatus incorporating the same for producing an even temperature over a cathode surface notwithstanding different heat losses from the variousparts of the surface, it will be understood by those skilled in the-art that these two methods may be combined to provide any degree of temperature compensation. Thus, there may not only be provided a single tube, a heater having unequally spaced turns in the case of a helical configuration, but also a heat shield about the cathode provided with gradated sizes of openings. As indicating the effectiveness of this combination, I have found that it is possible to elevate the temperature of the ends of a given cathode which normally runs at a much lower temperature than the middle, to a temperature C. or more above the middle. It is therefore apparent that by combining these features in the proper degree, a uniform temperature distribution can be obtained over the entire cathode. This uniformity of temperature is obtained without greatly increasing the temperature of the insulator on which the heater is wound.

What I claim as new and desire to secure by Letters Patent of the United States is: v

1. An indirectly heated cathode for an electric discharge device, said cathode comprising an electron-emitting member, the portions of which lose heat at different rates, and means including a heater and a heat shield for supplying heat to said portions and conserving the heat in proportion to the respective heat losses of the portions whereby the temperature of the member is maintained substantially uniform over its entire electron-emitting surface.

2. An electric discharge device comp-rising an envelope containing a plurality of electrodes including a thermionic cathode, supports for said electrode within the envelope, and means for heating and maintaining all portions of the oathode at substantially the same elevated temperature notwithstanding different rates of heat loss by the respective portions due to conduction through the cathods supports, said means including a source of heat which is gradated over the cathode in accordance with the degree with which heat is lost by conduction and including a shield which permits heat to pass through its respective portions only in proportion to the respective heat losses of the various portions of the cathode.

3. An electric discharge device comprising an envelope containing a plurality of electrodes including a thermionic cathode, supports for said electrode within the envelope, and means for heating and maintaining all portions of the cathode at the same elevated temperature notwithstanding different rates of heat loss by the respective portions due to conduction through the cathode supports, said means including a filamentary heater wound as a helix having its turns spaced closer together in those regions where the heat losses by conduction are greater and including a shield which permits heat to pass through its respective portions only in proportion to the respective heat losses of the various portions of the cathode.

4. An electric discharge device comprising an envelope containing a plurality of electrodes including a thermionic cathode, supports for said electrodes within the envelope, the cathode supports extending from one end of the cathode to the envelope, means for heating and maintaining all portions of the cathode to substantially the same elevated temperature notwithstanding different rates of heat loss by the said respective portions due to conduction through the cathode supports, said means including a heat shield and a filamentary heater wound as a helix with the turns spaced different distances apart, the turns nearer the end of the cathode from which the support rods extend being closer than the turns nearer the remaining portions of the oathode, said shield being perforated and permitting heat to pass through the respective perforations only in proportion to the respective losses of the various portions of the cathode.

5. An indirectly heated cathode for an electric discharge device, said cathode comprising an electron-emitting member, portions of which lose heat at different rates, a heater for said member, and means including a heat shield for controlling the heat radiated by the said respective portions in accordance with their respective heat losses whereby the temperature of the member is maintained substantially the same over its entire electron-emitting surface.

6. An indirectly heated cathode for an electric discharge device, said cathode comprising an electron-emitting member, portions of which lose heat at different rates, a heater for said member, means for controlling the heat radiated by the said respective portions in accordance with their respective heat losses, said means including a heat shield surrounding said member and having openings through which heat is radiated,

the size of the respective openings being dependent on the respective amounts of heat radiation necessary to maintain the temperatures of the said respective portions substantially the same notwithstanding difierences in the heat losses.

7. An electric discharge device comprising an envelope containing a plurality of electrodes including a thermionic cathode, supports for said electrode within the envelope, and means for heating and maintaining all portions of the cathode at substantially the same elevated temperature notwithstanding different rates of heat loss by the respective portions due to conduction through the cathode supports, said means including a heat shield having openings of difierent sizes for the egress of electrons and through which heat is radiated in different amounts.

8. An electric discharge device comprising an envelope containing a plurality of electrodes including athermionic cathode, supports for said electrode within the envelope, and means for heating and maintaining all portions of the cathode at substantially the same elevated temperaturenotwithstanding different rates of heat loss by the respective portions due to conduction through the cathode supports, said means including a heat shield having openings of different sizes for the egress of electrons, theopenings of the smaller sizes being in the region where the heat losses by conduction are the greater.

9. An electric discharge device comprising an envelope containing an indirectly heated cathode including a helical filamentary heater and an electron-emitting member in heat-receiving relation therewith, support rods secured to one end of the cathode for mounting the cathode in the envelope, a leading-in conductor secured to the same end of the cathode as the support rods, said rods and conductor permitting the escape of heat from said member by conduction, a heat shield surrounding said member, said shield having openings of different sizes through which heat is radiated, the sizes of the respective openings being determined by the necessary amounts of heat to be radiated by the respective portions of said member in order to compensate for the difierent rates of heat lost by the said portions of the member by conduction through said rod and leading-in conductor.

10. An electric discharge device comprising an envelope containing a plurality of electrodes including a thermionic cathode, supports for said electrodes Within the envelope, the cathode supports extending from one end of the cathode to the envelope, means for heating and maintaining all portions of the cathode at substantially the same elevated temperature notwithstanding different rates of heat loss by the respective portions due to conduction through the cathode supports, said means including a heat shield surrounding the cathode, said shield having openings of difierent sizes for the egress of electrons and through which heat is radiated, the openings in the region of the end of the cathode from which the support rods extend being smaller than the openings in the remainder of the heat shield.

LE ROY E. RECORD. 

